Method for solid state growth of iron single crystals

ABSTRACT

LARGE SINGLE CRYSTALS OF IRON ARE GROWN FROM A WELL ANNEALED, UNIFORMLY FINE-GRAINED SPECIMEN WHICH HAS BEEN GIVEN A SMALL AMOUNT OF PLASTIC STRAIN AND THEN HEATED TO INDUCE RECRYSTALLIZATION AT ONE SITE IN THE SPECIMEN. THE RECRYSTALLIZATION NUCLEUS IS ENLARGED AT THE EXPENSE OF ADJACENT STRAINED, FINE-GRAINED CRYSTALS BY REPEATEDLY HEATING THE SPECIMENT TO A TEMPERATURE ABOVE ABOUT 750* C. BUT BELOW ABOUT 910*C, FOR A PERIOD OF MINUTES AND THEN COOLING BELWO 750*C. THE DURATION OF HEATING AT THE ELEVATED TEMPERATURE OF EACH CYCLE IS SUCH THAT SUBSTANTIAL CRYSTAL GROWTH ON THE ORIGINAL NUCLEUS OCCURS BUT ADDITIONAL NUCLEATION TAKES PLACE IN THE REST OF THE IRON SPECIMEN.

Sept. 26, 1972 METHOD FOR SOLID STATE GROWTH OF IRON SINGLE CRYSTALS TEMPERATURE- C D. J. BAILEY ET AL 3,694,269

Filed NOV. 13, 1970 TIME INVENTORS flona/a JBai/ey (5 BY (far/ 6. Brewer ATTORNEY United- States Patent 3,694,269 Patented Sept. 26, 1972 US. Cl. 1481.6 3 Claims ABSTRACT OF THE DISCLOSURE Large single crystals of iron are grown from a well annealed, uniformly fine-grained specimen which has been given a small amount of plastic strain and then heated to induce recrystallization at one site in the specimen. The recrystallization nucleus is enlarged at the expense of adjacent strained, fine-grained crystals by repeatedly heating the specimen to a temperature above about 750 C. but below about 910 C. for a period of minutes and then cooling below 750 C. The duration of heating at the elevated temperature of each cycle is such that substantial crystal growth on the original nucleus occurs but additional nucleation takes place in the rest of the iron specimen.

This invention relates to a method of growing iron single crystals. More particularly, it relates to a cyclical heating method of growing a large single iron crystal from a recrystallization nucleus at the expense of strained neighboring crystals with little risk of nucleating the growth of other adjoining crystals.

The preparation of single crystals of many elements, including iron, is desirable from both commercial and research standpoints. Single crystals of many materials have desirable electrical and magnetic properties which make them useful in commerce. Large single crystals are also useful to researchers for a number of purposes, including their utility in basic studies on matter in the solid state.

Single crystals of some compositions may be prepared from a melt by controlled solidification techniques. However, a few metals, for example, iron, plutonium, titanium, uranium, zirconium, and many of their alloys, undergo one or more allotropic transformations between their melting point and room temperature. It is exceedingly difiicult to control or suppress these transformations, and single crystals of such materials, when grown from the melt, generally revert to polycrystalline form during cooling to room temperature. To overcome this difficulty the prior art has endeavored to develop solid state controlled recrystallization techniques wherein the growth temperature of the single crystal is below that of the otherwise intervening allotropic transformations.

Some prior art solid state crystal growth processes are based on the strain-anneal technique. In this procedure a well annealed, uniformly fine-grained specimen is given a small amount of plastic strain. It is then suitably heated to induce recrystallization at one site in the specimen. The specimen is then further heated in an attempt to cause the recrystallization nucleus to enlarge at the expense of the surrounding strained crystals until the entire specimen is consumed by the growing crystal. The perfection and limiting size of the resulting crystal is dependent upon how well other competing nucleation sites are suppressed. In practice, nucleation usually simultaneously occurs in several regions of the specimen and successful growth of a single crystal, free of occluded or adjoining crystals, is not attained.

Previous attempts to suppress unwanted nucleation invoked the concept of introducing a large, sharp temperature gradient in the polycrystalline specimen to be treated. The growth site region of the specimen is at a high temperature to promote growth on the recrystallization nucleus. In the case of iron this temperature is typically 850 to 875 C. An attempt is made to maintain that portion of the polycrystalline specimen removed from the growth area at a much lower temperature so that no nucleation takes place in the strained fine-grained crystals. Typically, the specimen or heat source is moved so that the high temperature region advances along the specimen at a rate of about 0.25 to 3 cm. per hour. Because crystal growth occurs more rapidly then nucleation, a moving gradient favors the continued growth of an existing crystal and inhibits the formation of fresh nuclei in cooler regions where nucleation is generally less favorable. However, the large, sharp temperature gradient is difficult to maintain, partly because of the high thermal conductivity of the iron specimen, and the successful growth of a single crystal by this method is frequently terminated by the random nucleation of several grains more favorably oriented for rapid growth. When complete growth is attained large parasitic grains usually remain, limiting the amount of the crystal that is suitable for use.

Accordingly, it is an object of the present invention to provide a method of growth single crystals of iron in the solid state which provides a high probalility of successfully obtaining nearly perfect, large single crystals.

It is another object of the present invention to provide a method of growing large, nearly perfect single crystals of iron in the solid state which does not require the establishment of a large, sharp temperature gradient in the specimen to prevent unwanted nucleation.

It is a further object of the present invention to provide a method of growing large single crystals of iron in the solid state from a suitably annealed, fine-grained specimen by repeatedly heating and cooling the specimen under conditions which promote growth on a chosen nucleation site and prevents unwanted recrystallization nucleation in other portions of the specimen.

In accordance with a preferred embodiment of our invention, these and other objects are accomplished by first providing an annealed, uniformly fine-grained and critically strained iron specimen containing a suitably oriented seed crystal. This iron workpiece may be in any desired form, such as a strip or rod. It is heated for a period of several minutes at a temperature of 850 to 875 C. and then cooled well below 750 C. This heating period, typically about two to ten minutes at the high temperature, promotes crystal growth on the seed crystal, but the period is too short for nucleation of any new seed crystals in the specimen. The heating and cooling procedure is repeated many times. The growing crystal consumes all of the original fine grains and, finally, the entire specimen consists of a large, nearly perfect iron crystal. The critical features of our process are the length of time at which the specimen is maintained at the crystal growth temperature and the repeated heating and cooling procedure to obtain a large single crystal without forming other small ones.

Other objects and advantages of our invention will become more apparent from the detailed description thereof which follows. Reference will be made to the drawings, in which:

FIG. 1 is a graph depicting the variation of temperature with time in the practice of a preferred embodiment of our process; and

FIG. 2 is a graph illustrating the known phenomenon that the formation of stable recrystallization nucleation sites in a strained crystalline material at a suitable temperature requires a substantial incubation period.

In preparing specimens for the practice of our process substantially pure iron will have been formed by cold working into a desired predetermined shape, such as a rod, strip or the like. When the specimen has been formed to predetermined dimensions it is annealed to produce a completely recrystallized, uniformly fine-grained size material. In accordance with the practice of our process we consider fine grains to be of 0.006 inch average diameter or less. This is equivalent to an ASTM #4 grain size or smaller when determined in accordance with ASTM Micrograin Size Standard E-l 12. It is necessary that the specimen be fully recrystallized and all residual stress relieved so that there are no active recrystallization nuclei present. The anneal varies with the cold work history of the material and typically will involve heating the specimen for two hours or more at 820:20 C.

The fully annealed specimen is then slowly plastically strained to introduce strain energy into the polycrystalline body. It is strained slowly to avoid the multiple Liiders band formation which is experienced in limited stretching of iron and low carbon steel specimens if the material has not been specially treated to temporarily preclude the band formation. The total strain should not be so great as to promote gross recrystallization of the specimen during heating and subsequent crystal growth at temperatures in excess of 800 C. A small total strain of up to 3% to 8% is suitable, while a total strain of about 4% is preferred. When a strip or rod specimen is being prepared the strain can be introduced by tensile stretching in an Instron Universal Testing Machine.

The specimen is then deformed more severely in one location, such as the end. This may be accomplished, for example, by striking the specimen with a punch. If the specimen was initially strained in an Instron machine the gripping members of the machine will have satisfactorily defromed ends of the specimen. In this instance one end is removed. The further deformed portion of the specimen is then placed in a furnace, preferably at 850 to 875 C. for fifteen to thirty minutes to induce the formation of a number of recrystallization seed crystals in the more heavily worked site. The seed crystals are examined, particularly with respect to their orientation. A crystal of desired orientation is selected and the others are trimmed or etched away from the specimen. In the case of a rod or strip specimen the seed crystal will be located at one end and connected to the rest of the specimen by a neck of fine-grained material. The specimen is then ready 'for treatment by our process.

Our process takes advantage of the fact that an existing recrystallization nucleus will immediately start to grow at the expense of surrounding crystals at a suitable elevated temperature. In the case of the application of our process to pure iron, the suitable elevated temperature range for grain growth in a specimen which has been strained up to 8% is above about 750 C. but below the phase transformation temperature of 910 C. At these same temperatures additional recrystallization nuclei will form in the fine-grained portion of the specimen if the specimen is maintained at the elevated temperature for a sufiiciently long period of time. However, there is a very definite incubation period, as shown in FIG. 2, in which no stable recrystallization nuclei are formed. Further heating at a temperature above the recrystallization temperature will thereafter result in the formation of a rapidly increasing number of stable recrystallization nuclei.

This incubation period varies substantially and depends largely upon the purity of the material and the amount of strain energy in the specimen. We have found that the incubation period is about two to ten minutes for samples which are first annealed and then strained about 4%, as set forth above. So long as our specimens are heated for a couple of minutes, up to about ten minutes, crystal growth on the seed crystal is obtained without the formation of additional nuclei. The specimen must then be cooled below the recrystallization temperature of about 750 C. to pre- 4 vent the formation of additional recrystallization nuclei. After cooling we have found that it may be immediately reheated above 750 C. to about 850 to 875 C. Crystal growth is then permitted to continue for a period of a couple of minutes, up to about ten minutes, and the sample is cooled again below 750 C.

This procedure is depicted schematically in FIG. 1. The specimen is repeatedly heated above 750 C., preferably to 850 to 875 C., for up to about ten minutes and then cooled below 750 C. The procedure is repeated many times until a single crystal of desired size is obtained or until the whole specimen has been converted to a nearly perfect single crystal.

A better understanding of the process will be obtained by a specific example thereof.

A strip of substantially pure iron was provided by cold working until it had dimensions of 20 cm. x 2 cm. x 1 mm. It was fully annealed at a temperature of 820 C. to remove the effects of the cold working and to provide a uniformly fine-grained microstructure. The strip was then placed in an Instron Universal Testing Machine and slowly stretched to a total tensile elongation of 4%. The strain was undertaken slowly to avoid the formation of mutiple Liiders bands. In this initial experiment about 2 cm. of stock were sawed off from each end of the strip to remove those portions which were more severely deformed by the grips of the Instron machine. The ends of the strip were then etched in 50% nitric acid to dissolve metal subjected to the cold work of the sawing operation. At this point it was presumed that all effects of excess cold working due to the cutting operation had been removed.

To determine whether the residual strains of cold working had been completely removed and that there were no active recrystallization nuclei present in the strip, it was briefly heated in a furnace at 875 C. The furnace was of open ended design and arranged and constructed to maintain a hydrogen atmosphere around the iron workpiece. The strip was advanced through the furnace at a rate of 350 cm. per hour. After the first pass it was cooled to about 300 C. and passed through the furnace a second time. This heating and cooling was then repeated a third time. The strip was then examined to determine whether it was stable to recrystallization under these heating conditions. It was etched over all of its surface in 10% nitric acid and examined visually for the presence of large grains or for recrystallization nuclei. No change in the original fine-grained structure was noted and the strip was deemed suitable for further processing into a single crystal of iron.

One end of the strip was struck with a punch. The strip was again passed through the furnace three times at 875 C. as described above. After this repeated heating and cooling the more heavily deformed region was etched in 10% nitric acid and examined visually and by X-ray diffraction. Three recrystallization seeds about 5 mm? in area had formed in the heavily deformed region which had been struck by the punch. A seed crystal was chosen whose axis was most closely aligned with the longitudinal direction of the strip. The other seed crystals were cut from the strip. The selected seed was left connected to the strip by a neck of fine-grained polycrystalline material. The seed was rotated and twisted to bring its [110] axis in alignment with the longitudinal axis of the strip. The seed and neck were briefly etched with 50% nitric acid to remove the metal subjected to the cold work by cutting of the undesired seeds.

The strip, seed end first, was again passed through the furnace, heated at 875 C. and containing a hydrogen atmosphere. The strip was moved at a rate of 350 cm. per hour. Each region of the strip was at 850 to 875 C. for 2.2 minutes before passing out of the furnace and rapidly being cooled in hydrogen to a temperature below 300 C. This procedure of heating the strip above 850 C. by continuously moving through the open-ended furnace and subsequently cooling it below 300 C. was

repeated about 100 times. At the completion of the last heating and cooling cycle the entire strip had been converted to a substantially perfect single crystal of iron.

In the practice of our process the permissible length of time at which the iron workpiece is maintained at the elevated crystal growth temperature depends upon the stability of the fine-grained portion of the piece to recrystallization. The more completely all effects of previous cold work have been removed before the formation of the seed crystal at the desired site and subsequent heat treatment, the longer the strip may be maintained at the growth temperature. Of course, the longer that the workpiece can be maintained at the growth temperature, the fewer the number of heating and cooling cycles required to produce a single crystal of given size. The rate of growth of a particular seed crystal, as is known, depends upon its orientation with respect to the matrix. Some crystal orientations grow faster than others. Therefore, the required number of passes to produce a single crystal of a given size by our process is also dependent upon the desired orientation of the single crystal to be produced.

A number of diiferent heating and cooling procedures may be employed in carrying out our process. In preparing long single crystals of iron in the form of a strip or rod we prefer to quickly and continuously move the strip through an open-ended furnace as described above in the example. This procedure is particularly suitable for treating long workpieces which may not readily fit in an available furnace. In another embodiment of the invention the specimen is simply placed entirely within a furnace until it has reached and been maintained at the desired crystal growth temperature for a suitable period of minutes, and then is completely withdrawn from the furnace until it is cooled below 750 C. The specimen is repeatedly introduced into the furnace for suitable heating and withdrawn from the furnace for cooling until the desired crystal growth is obtained. It is preferred that the heating and cooling in all instances be under a nonoxidizing atmosphere.

In a further embodiment of the invention the specimen is heated by high intensity infrared lamps. The lamps are simply turned on and off in a programmed fashion to obtain the alternate and repeated heating and cooling cycles of our process. It is also possible to heat the specimens by passage of an electrical current therethrough. Control of the duration and spacing of the electrical current impulses accomplishes the heating and cooling steps of our process.

While our invention has been described in terms of certain preferred embodiments thereof, it is recognized that other forms of the invention could readily be adapted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.

What is claimed is:

1. A method of preparing large single crystals of iron comprising:

providing an annealed, uniformly fine-grained specimen of iron, subjecting said specimen to a small amount of plastic strain up to about 8%,

annealing said specimen to induce recrystallization and the production of a growth nucleus at a random site in said specimen,

heating at least that portion of said specimen in the region of said growth nucleus at a temperature above about 750 C. for a period of minutes sufficient to induce growth of said nucleus into a large crystal at the expense of the neighboring fine crystal grains,

cooling said specimen below 750 C. before additional growth nuclei are formed in said specimen,

and repeatedly heating and cooling said specimen until a single crystal of desired size is obtained with said specimen.

2. A method of preparing large single crystals of iron comprising:

providing a completely recrystallized, uniformly finegrained specimen of iron,

slowly straining said specimen to a total elongation of about 3% to 8%,

further straining said specimen at a predetermined site therein,

annealing said specimen to induce recrystallization and the formation of a growth nucleus at said further strained site,

heating said specimen to a temperature above about 750 C. but below about 910 C. for a period of minutes to induce growth of said nucleus into a large crystal at the expense of adjacent fine-grained crystals but without forming nucleation sites in the finegrained portions of said specimen,

thereafter cooling said specimen below 750 C.,

and repeatedly subjecting said specimen to said heating and said cooling until a single crystal of desired size is obtained therein.

3. A method of preparing large single crystals of iron comprising:

providing a recrystallized, uniformly fine-grained specimen of iron,

subjecting said specimen to a small amount of plastic strain up to about 8%,

further straining said specimen at a predetermined site therein,

annealing said specimen to induce recrystallization and the formation of a growth nucleus at said further strained site,

moving said specimen into and out of a furnace, which is maintained at a temperature in the range above about 750 C. and below 910 C., at a rate such that said specimen is at a temperature in said range for a period of minutes sufiicient to induce growth of said nucleus into a large crystal at the expense of neighboring fine crystal grains but without forming stable nucleation sites in the fine-grained portions of said specimen,

and repeatedly moving said specimen into and out of said furnace, thereby repeatedly alternately heating said specimen above 750 C. for a said period of minutes and cooling said specimen below 750 C. before additional growth nuclei are formed, until a single crystal of desired size is obtained in said specimen.

References Cited UNITED STATES PATENTS 1,738,307 12/1929 McKeehan 1481.6 R 3,027,281 3/1962 Osborn-let a1. 1481.6 R 1,560,335 11/1925 Czochralski 1481.6 R X GEORGE T. OZAKI, Primary Examiner US. Cl. X.R. 148-12 

